c++ - When I want to declare new members for a Struct,some trouble I don't understand

I init a Struct member like this below:

struct MyStruct
            {

 int member_a;

};

int
            main(){

MyStruct s;//method 1

MyStruct *
            ps;//method 2

return
            0;
}

What's
the difference between method 1 and 2 ??Why do someone use method 1 and some others use
method2?

Answer

I generally use method_2. In data structures
like binary trees, if I have a pointer to a struct_node, say temp_pointer, and now I
need to change this to its left_child, I can simply make the pointer point to the
left_child. Now, if I need to change some value in the left_child, I can simply change
that value in the node to which temp_pointer points. This won't be possible with
method_1. As there, we would have a separate copy of the left_child instead of a pointer
to the left_child (a separate copy will have same values, but different address).
method_1 will not change the value in the original node (i.e. the left_child), but only
in the copy.

Also, suppose we have a
mystruct_pointer and another temp_pointer. We can compare both (mystruct_pointer ==
temp_pointer), and check whether they point to the same node or not. This won't be
possible with method_1.

Remember, this method_2
only declares a pointer to mystruct type. To actually create mystruct type, you will
have to allocate memory using malloc or calloc.